Optical signals are suitable for high-speed, high-capacity signal transmission and have already been implemented in long-distance trunk communication systems. Furthermore, due to the increase in the speed of signals also for information devices such as computers, optical signals have already been implemented among such devices, and the introduction of optical signals within devices and within boards is on the horizon.
Optical fibers are excellent in terms of performance and price as wiring members that connect separate locations. However, it is desirable for portions that process optical signals such as optical transceivers, optical couplers, optical splitters, and arrayed waveguide gratings (AWG) and so forth to be formed as optical waveguides.
In addition, silicon photonics is also recently being employed. This has the advantage in that it is possible to form the same functions in very small areas by using semiconductor manufacturing processes to finely process silicon. There are limited uses for individual optical waveguide components formed on such substrates, but by connecting with the aforementioned optical fibers it is possible for optical signals that are processed by optical waveguides to be transmitted to target locations.
Together with improvements in the functions and the degree of integration of such optical waveguide components, the number of connecting optical fibers has increased remarkably. Ordinarily, optical waveguides and optical fibers are connected by using components that are referred to as fiber arrays in which optical fibers are adhesively fixed in V-grooves that are formed at regular intervals (for example, see Japanese Laid-Open Patent Publication No. 2011-247913).
In order to connect an optical waveguide and an optical fiber with low loss, the positional relationship of these has to be precisely controlled. Positional precision of approximately 1 μm is desired in single mode, and even in multi-mode positional precision within several μm is desired. A fiber array in which optical fibers have been aligned has the merit of it being possible to connect a large number of optical fibers at the same time; however, in order to meet the target positional precision, precise alignment not only with regard to XYZ triaxial movement but in a total of six axes including the rotation of the axes is desired. In particular, precision in the directions of rotation becomes remarkably strict as the number of optical fibers aligned in the fiber array increases.
This kind of problem regarding alignment precision is widely known, and there have been a large number of proposals for achieving both positional precision and ease of optical fiber connection work by forming, in optical waveguide substrates, V-grooves for the alignment of optical fibers (for example, see Japanese Laid-Open Patent Publication No. 2006-119627, Japanese Laid-Open Patent Publication No. 08-313756, Japanese Laid-Open Patent Publication No. 2005-308918, Japanese Laid-Open Patent Publication No. 01-126608, and Japanese Laid-Open Patent Publication No. 2004-151391).
In order to form V-grooves, a single-crystal silicon substrate is used as the material for an optical waveguide substrate. By performing anisotropic etching on this single-crystal silicon substrate, it is possible to obtain V-grooves having a precise shape that is determined by the angles of the crystal planes. By then using photolithography techniques to specify the etching pattern, it is possible to form V-grooves having accurate dimensions proximate to the core of an optical waveguide.
However, a problem arises as to whether to form the optical waveguide or the V-grooves first. If the V-grooves are formed first, a core of several μm to several tens of μm is formed in the end section of very large grooves of 100 μm or more; however, it is difficult to form the core without there being any deformation in the shape.
Consequently, in order to avoid this kind of problem, a method has been proposed in which a core is formed in a state where preformed V-grooves are temporarily filled with a resin (for example, see Japanese Laid-Open Patent Publication No. 2006-119627). Alternatively, a method has been proposed in which a core is formed in a state where a lid is placed on preformed V-grooves (for example, see Japanese Laid-Open Patent Publication No. 08-313756).
However, the method in which these V-grooves are temporarily filled has the drawbacks that the method is labor intensive and the effect thereof is insufficient. For example, when resin filling is employed, complete smoothness is unlikely and the shape of the core becomes deformed. On the other hand, when covering with a lid, if the positional precision with which the lid is placed and the thickness of the lid and so forth are not controlled, there is a problem in that there is a high possibility of the shape of the core being deformed by the lid.
Contrastingly, a method has been proposed in which V-grooves are formed later (for example, see Japanese Laid-Open Patent Publication No. 2005-308918). However, boiling for a long period of time in a highly concentrated alkaline solution has to be carried out for the anisotropic etching of silicon, and there are not many optical waveguide materials that can withstand this process. Specifically, although there are silica (SiO2)-type optical waveguides, even these become damaged if strong protective measures are not taken.
Furthermore, another drawback of employing anisotropic etching of silicon is that the wall surfaces of the optical waveguide core also become inclined, and it is not possible for the optical fibers to be brought into close proximity with the core. In order to avoid this problem, forming vertical grooves by separate processing (for example, see Japanese Laid-Open Patent Publication No. 01-126608), and forming optical fibers into a shape corresponding to the inclined surface (for example, see Japanese Laid-Open Patent Publication No. 2004-151391) have been proposed.
However, for the former proposal, two types of processing have to be used, and therefore the forming step takes time. On the other hand, the latter proposal has a problem in that as well as the tip-end processing of the optical fibers being difficult in itself, the optical fibers have to be arranged in the V-grooves with the rotational direction orientations thereof being aligned. As described above, the effect is considerable if V-grooves can be formed in an optical waveguide substrate; however, there has been a problem in that the forming is itself very difficult.